As a communication network carrying out the sending and receiving of optical transmission unit frames, there is Optical Transport Network (OTN) specified in G. 709 of the ITU-T Recommendation. The frame of this OTN network has a hierarchical structure as will be described hereinbelow. First, a client signal (SDH, Ethernet™, and the like) with added OPU overhead (OH) is called an Optical-channel Payload Unit (OPU). The definition of each byte of the OPU overhead varies with the client signal which is an OPU payload. Next, This OPU with added ODU overhead is called an Optical Data Unit (ODU). Then, this ODU with added OTU overhead and Forward Error Collection (FEC) becomes an Optical Channel Transport Unit (OTU) frame. Further, when k is added such as OTUk and ODUk, they refer to the number k or the kth OTU and ODU.
The OTU frame is composed of 4080 bytes in four rows. The bytes of each row are specified by the number of columns 1 to 4080. The OTU/ODU overhead is positioned in the columns 1 to 14. The OPU overhead is positioned in the columns 15 and 16. The OPU payload is positioned in the columns 17 to 3824. The FEC is positioned in the columns 3825 to 4080.
FIG. 10 shows a definition of the OTU/ODU overhead. Each symbol in the diagram has the following meaning. FAS indicates a frame alignment signal. MFAS indicates a multi-frame alignment signal. SM indicates section monitoring. GCC0 to GCC3 indicate general-purpose communication channels. RES indicates a reserved state. TCMACT indicates tandem connection monitoring activation. TCM1 to TCM6 indicate tandem connection monitoring. FTFL indicates failure type and failure location. PM indicates path monitoring. EXP indicates experimental usage. APS/PCC indicates automatic protection switching/protection communication channel. The bytes disposed in the columns 8 to 14 of the row 1 constitute the OTU overhead. The bytes disposed in the columns 1 to 14 of the rows 2 to 4 constitute the ODU overhead.
The OTU overhead provides various monitoring functions. As one of them, it is the function for detecting transmission error of OTU frames and, for this function, BIP-8 is defined in the second byte of SM. In the same manner, as one function of the ODU overhead, there is a function for detecting transmission error of ODU and, for this function, BIP-8 is defined in the second byte of PM and TCM1 to TCM6.
BIP-8 is the abbreviation of bit interleaved parity of 8 bits. BIP-8 calculates the parity eight times in such a manner as for the first bit, the second bit and the like of each byte included in a calculation range. In G. 709 of the ITU-T Recommendation, as shown in FIG. 11, the portion of OPU data (the area combining the OPU payload and the OPU overhead: the columns 15 to 3824 in the rows 1 to 4) is taken as the calculation range. Further, the value of BIP-8 calculated for the frame i is inserted into a predetermined place in the frame i+2 which is the second succeeding frame after the frame i (the second byte of SM for the OTU overhead; the second byte of PM and TCM1 to TCM6 for the ODU overhead).
The following patent document 1 discloses a transmission system carrying out error monitoring by the BIP-8 calculation as described hereinabove. Further, the following patent document 2 discloses an optical transmission network.
[Patent Document 1] JP 2933479
[Patent Document 2] JP 2003-188919 A
As described hereinabove, in G. 709 of the ITU-T Recommendation, the portion of OPU data (also to be described as payload information hereinbelow at times) is taken as the calculation range of BIP-8. Therefore, even though transmission error occurs in the portion of the OTU/ODU overhead, it cannot be detected by BIP-8.